amateur detective

If I were to call someone an amateur in my own language, it wouldn't be meant in a positive way. A general practitioner, for example, is a good example. If they're an amateur, or rather, amateurish, it's not a good sign, and you'd better stay away.
I don't believe detective work is a private profession in our country, let alone that you'd encounter an amateur detective. Now that I think about it, I wonder why that is. Nobody has much regard for the average police detective; they certainly don't investigate what actually happened. Suppose it's not excuse about a shortage of manpower. In that case, they deliberately make stupid mistakes, let cases lapse in time, or shrug their shoulders, leaving the judge to sort things out (naturally, with half their report in hand). Plenty of celebrities, like John Does, have been murdered and are kept quiet about. Perhaps it wouldn't be so crazy to have more private detectives who are inclined to get their jaws stuck like a pit bull.
Peter R. de Vries could be called an amateur detective. Of course, he wasn't, and... he preferred to use the name "crime reporter" while he tried to solve the cold cases. Something he often succeeded in doing.